1. Field of the Invention
This invention relates to a noncontact type magnetic head wear-rate measuring apparatus suitable for use in a rotating drum device using a magnetic head, such as a video tape recorder, a data recorder or the like.
More specifically, the present invention relates to a noncontact type magnetic head wear-rate measuring apparatus wherein a magnetic sensor is disposed in a state placed in noncontact with a rotating magnetic head device and the rate of wear of a magnetic head can be measured in a noncontact state and with high accuracy according to a variation in the total magnetic resistance between the magnetic head and the magnetic sensor.
2. Description of the Related Art
In AV devices each using a rotating drum device with a magnetic head mounted thereon, such as a video tape recorder (VTR), a data recorder, a digital audio tape recorder (DAT), etc., the magnetic head is relatively driven in a state of being in contact with the magnetic tape. Therefore, a tape sliding portion of the magnetic head wears out due to the running of the magnetic tape over a long period of time.
When the rate of wear thereof reaches several tens of microns, a region (head depth) for forming a magnetic head gap disappears in the case of a normal magnetic head. Therefore, there may be cases in which when the magnetic head wears till the instance preceding the complete disappearance of the head depth, such a magnetic head interferes with recording and reproduction. Further, when the head sliding portion wears out until the head depth completely disappears, the worst case occurs and hence a signal cannot be recorded and reproduced.
Since, in this case, a signal reproduced from the magnetic tape is brought to zero when the rate of wear of the head has reached several tens of microns during signal reproduction, a malfunction in the magnetic head can be immediately recognized.
However, when the rate of wear of the head has reached several tens of microns during recording of a signal, there is the risk that the signal cannot be normally recorded and important information may be excluded. This abnormal condition cannot be confirmed unless the recorded signal is reproduced. Thus, when the AV devices are used in particular as for commercial use, such a situation must be avoided.
Therefore, particularly when the rate of wear of the head employed in each AV device for commercial use is monitored and the rate of wear of the head reaches a predetermined value, the AV device preferably warns a user of its fact so as to urge the user to perform its maintenance and inspection. It is therefore necessary to measure the rate of wear of the head. In this case, however, a contact type measuring apparatus or a noncontact type measuring apparatus is considered as this type of measuring apparatus.
In the contact type magnetic head wear-rate measuring apparatus, a measuring jig such as a measuring element or probe is mounted to a magnetic head to be measured so as to come into contact with the magnetic head. Therefore, there is a possibility that a tape sliding surface of the magnetic head to be measured has flaws or the magnetic head or the magnetic head will break in the worst case. The result of measurements varies depending on how to mount the measuring probe to the magnetic head and an influence exerted on measuring accuracy cannot be overlooked.
When the rate of wear of a magnetic head to be measured is measured by the noncontact type magnetic head wear-rate measuring apparatus, the present measuring apparatus does not cause such a problem as produced in the contact type magnetic head wear-rate measuring apparatus. The noncontact type magnetic head wear-rate measuring apparatus measures the rate of wear of the head using light. In this case, a laser beam or the like is used as the light. Since the laser beam must be focused onto a tape sliding surface of the magnetic head so as to be accurately applied onto the tape sliding surface, the layout, adjustments and the like of a laser optical system become so troublesome. The measuring apparatus itself increases in volume due to the use of the optical system and a manufacturer gets greatly nervous at the assembly of the measuring apparatus into the rotating drum device.
An object of the present invention is to provide a noncontact type magnetic head wear-rate measuring apparatus capable of measuring the rate of wear of a magnetic head in a noncontact state and measuring the rate of head wear with high accuracy.
According to one aspect of the present invention, for achieving the above object, there is provided a noncontact type magnetic head wear-rate measuring apparatus, comprising:
magnetic sensor opposed to a rotating magnetic head device with a magnetic head mounted thereon and disposed in a state placed in noncontact with the rotating magnetic head device so as to fall outside an angle at which a magnetic head tape is wound around the rotating magnetic head device;
means for measuring the rate of wear of the magnetic head in response to an output produced from the magnetic sensor; and
means for detecting a position of rotation of the rotating magnetic head device and supplying its detected output to the measuring means; and
wherein the measuring means measures the rate of wear of the magnetic head, based on a variation in oscillating frequency of the magnetic sensor.
The magnetic sensor is disposed at and fixed to a position which falls outside a tape lap angle. The magnetic sensor comprises an inverted U-shaped frame core and a detecting coil wound in a winding groove defined in the core.
The width of the winding groove is wider than a gap width of the magnetic head and is narrower than the width of the magnetic head.
Since the total value of magnetic resistance including a plurality of magnetic heads and a magnetic sensor at rotational positions where the magnetic heads are respectively opposed to the magnetic sensor, changes as the magnetic heads wear out, a variation in magnetic resistance is taken as a variation in inductance. Since the inductance is a part of oscillator circuit elements, the oscillating frequency thereof also changes with the variation in inductance. Since the rate of wear of each magnetic head and the change in oscillating frequency are correlated with each other, an oscillating frequency at the time the rate of head wear is zero (before the magnetic head is used), is stored. If a variation in the subsequent oscillating frequency is monitored, then the rate of head wear at the time of its measurement can be recognized. When the rate of wear of the head exceeds a predetermined wear rate (predetermined value), a warning is issued to a user. This can avoid beforehand an undesired state that a signal is not suddenly recorded during recording of the signal.
Each position (address) to be measured relative to the magnetic head is shifted little by little. Further, an oscillating frequency is determined each time its shift is made, and positions where the magnetic sensor and the magnetic head are completely opposed to each other, are determined from total data thereof. Thereafter, the position to be measured is specified according to the corresponding addresses indicative of the completely opposed positions and its measurement is executed.
When the plurality of magnetic heads are provided with steplike offsets defined relative to each other in the direction of rotation of the rotating magnetic head device, the size (thickness) of the magnetic sensor is selected so that a single magnetic sensor can cover the magnetic heads, i.e., the plurality of magnetic heads are all included within a magnetic gap of the magnetic sensor.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.